This invention relates to electric hybrid vehicles and, in particular, it relates to a combined series-parallel electric hybrid vehicle.
There are basically three types of electric propulsion systems known for vehicles. First, there is a pure electric drive vehicle. The pure electric drive vehicle has an electric motor which receives power from a main battery pack via a controller. The controller controls the speed of the electric motor. The major disadvantage of a pure electric drive vehicle is that the range is very limited and the vehicle must be stopped and connected to an energy source such as an electrical outlet in order to be recharged.
The second type of electric propulsion system for vehicles is a series hybrid system. There are three major components in a series system: (1) a generator; (2) an electric motor arranged in series; and (3) an engine powering the generator. Mechanical energy generated by the engine is converted to electrical energy by the generator and is then converted back to mechanical energy by the electric motor. Each process of conversion is afflicted with losses and subsequent reductions of efficiency which is a significant disadvantage of this type of system.
The main advantage of the series hybrid is that it is possible to operate the engine at a fixed operating point within its engine speed/torque map. This point can be selected so that the engine functions with the greatest efficiency or produces particularly low emissions. Nevertheless, the efficiency of the entire series hybrid drive system is less than satisfactory.
The third type of electric propulsion systems is the parallel hybrid system, as described, for example, in U.S. Pat. No. 5,081,365. Parallel hybrid propulsion systems generally have three component areas. (1) electrical storage mechanism, such as storage batteries, ultracapacitors, or a combination thereof; (2) an electric drive motor, typically powered by the electrical storage mechanism and used to propel the wheels at least some of the time; and (3) an engine, such as a liquid fueled engine (e.g. internal combustion, stirling engine, or turbine engine) typically used to propel the vehicle directly and/or to recharge the electrical storage mechanism.
In parallel hybrid systems, the electric drive motor is alternatively driven by mechanically coupling it to the engine. When coupled, the engine propels the vehicle directly and the electric motor acts as a generator to maintain a desired charge level in the batteries or the ultracapacitor. While a parallel hybrid system achieves good fuel economy and performance, it must operate in an on and off engine parallel mode. In this mode, the stop-and-go urban driving uses electric power and the engine is used to supplement existing electric system capacity. For long trips, when the battery for the electric motor could be depleted, the vehicle cruises on the small engine and the electric system will provide the peaking power.
The primary advantage of the parallel hybrid drive over the series drive previously described is improved efficiency (lower fuel consumption) in the engine, since the engine""s mechanical energy is passed directly on to the drive axle. The bulky generator is no longer required, thereby lowering both the cost and weight of the vehicle.
However, with extended stop and go urban driving, the battery pack will be often depleted and will need a charge in addition to the charge received from the electric motor. Or, the engine will be required to power the vehicle during the stop and go driving period thereby eliminating most beneficial effects of such an electric system. Therefore, the vehicle with a parallel system has limited inner city driving capabilities and range.
Due to the innate, but separate, advantages of both the series and the parallel drives, a method of combining series and parallel systems has been invented. In the present invention, the engine has an alternator or generator connected directly to the engine""s drive shaft by some mechanism, for example, a fan belt. Generally, alternators or generators are used to charge the battery of a vehicle""s accessory systems, such as the light, fans, etc. These systems typically operate on twelve (12) volts. However, the inventors of the present invention realized that the alternator is very capable of high current/high voltage output, ranging from, but not limited to, approximately ten (10) volts to in excess of one hundred fifty (150) volts. In standard applications, such as vehicle accessory systems, voltage output is regulated to approximately fourteen (14) volts. Implementation of the present invention allows for efficient usage of the upper limits of the alternator""s output capacity. Voltage output can be controlled by a central process controller, which directs excess current to the parallel system vehicle""s main storage battery pack. Voltage output can be varied to the appropriate levels by regulating the field current, among other methods of control.
The current flow, for example to the twelve (12) volts accessory battery, or to the hybrid vehicle""s main storage battery, can be controlled simply by solid state switching mechanism. An automatic, selectable voltage output of the alternator will also be controlled by automatic mechanism via the process controller.
An alternative method of control is to set the alternator to a continuous high voltage level, matching that of the hybrid""s main battery pack. A switching power supply would then channel generated current into the main battery pack, or into the vehicle""s twelve (12) volt battery. The switching power supply has the ability to reduce voltage to the appropriate level, based upon which electrical system is being fed.
This arrangement eliminates the main disadvantage of conventional parallel hybrid designs as used in a vehicle. It has been found that at slow speed, such as stop and go urban driving, the parallel system will allow the main storage battery pack to deplete its energy below a comfortable and usable level of charge. A series hybrid system is more adaptable to urban driving because it constantly funnels limited amounts of electrical energy back into the system""s battery pack. The main negative of a series hybrid system is that it does not permit an adequate charging level to sustain the high energy demand associated with long term, high speed driving. The present invention prevents depletion of the battery pack by better utilizing the existing component structure typically associated with parallel hybrid systems.
Prior hybrid propulsion systems were typically capable of operating in one or more of the following modes (but none were capable of operating in a choice of all of them): (1) a series hybrid, which is plugged in for recharge, and which uses the engine as a xe2x80x9crange extenderxe2x80x9d when the electrical storage mechanism are depleted, and/or (2) a series hybrid which runs the engine in order to recharge its own electrical storage mechanism, typically via a generator/alternator, and/or (3) a parallel hybrid, which is plugged in for recharge, and which uses the engine and/or the electric motor either separately or in unison, depending upon conditions, circumstances, and the process controller, in order to directly power the vehicle, and/or (4) a parallel hybrid similar to the one described in (3), directly above, but which recharges its own electrical storage system via the engine and, typically, a generator/alternator (see U.S. Pat. No. 5,081,365). Each of these modes has its benefits and drawbacks, depending on circumstances, and the industry is involved in debate over which system is the most promising.
The purpose of the series-parallel functionality is to overcome problems inherent to either concept when employed individually. The advantages are increased range in the urban driving mode and a secondary method of range extension in highway mode without significantly increasing the bulk or cost of the base parallel system. In addition, the control of the operation of the drive motor is more versatile and efficient.